Device for determining fuel quality and corresponding method

ABSTRACT

The invention relates to a device for determining the quality of fuel for an internal combustion engine, comprising a pressure sensor ( 1 ) for determining the pressure (p) inside a fuel tank and/or comprising a temperature sensor ( 4 ) for determining the temperature (T) inside a fuel tank. According to the invention, an evaluation unit ( 5 ) is provided for determining a quality value (Q), which depicts die quality of the fuel, based on the temperature (T) and/or the pressure (p) inside the fuel tank. Said evaluation unit is connected on the input side to the pressure sensor ( 1 ) and/or to the temperature sensor ( 4 ).

CLAIM FOR PRIORITY

This application is a national stage of PCT/DE03/01168, published in theGerman language on Apr. 9, 2003, which claims the benefit of priority toGerman Application No. DE 102 17 379.6, filed on Apr. 18, 2002.

TECHNICAL FIELD OF THE INVENTION

The invention relates to a device for determining the quality of fuelfor an internal combustion engine.

BACKGROUND OF THE INVENTION

The quality of the fuel used to drive internal combustion engines may besubject to manufacturing tolerances, for example the fuel may containvarying proportions of long-chain hydrocarbons.

It is therefore a well-known procedure to determine the quality of thefuel used and to take this into account in the fuel injection process inorder to compensate for the quality variations as far as possible.

At present, variations in fuel quality are detected using astarting-quantity adaptation technique or the uneven-running method, andcompensated for by varying the injection periods. In these knownmethods, the increase in speed when starting the internal combustionengine or the speed variations at start-up are evaluated. If theincrease in speed or the speed variations lies/lie outside an allowedpreset confidence band the injection period is corrected accordingly.The correction amount is calculated relatively imprecisely, however, sothat sometimes the correction proves too great. In particular, given a“poor start”, the injection period is changed so as to make the fuel-airmixture richer, which means that the fuel-air mixture may be too richafter filling up with a very good quality fuel. As a result, theinternal combustion engine starts badly or not at all.

Another disadvantage of this method for determining the fuel quality isthe relatively low accuracy, because the increase in speed when startingthe internal combustion engine does not depend solely on the fuelquality but is also affected by other parameters such as the internalfriction of the internal combustion engine.

DE 40 27 947 A1 describes various ways in which one can take intoaccount fuel quality variations. One way is to measure the controldeviation in the lambda control circuit after each tank fill of thevehicle driven by the internal combustion engine, and to vary anadaptation value so as to reduce the control deviation to zero. Thismethod has the disadvantage that it only works when the lambda controlsystem is enabled. This is specifically not the case, however, when theinternal combustion engine is cold. Other methods are proposed thatensure that, even when the internal combustion engine is cold, theinternal combustion engine is still able to run when the fuelcomposition changes sharply at tank fill-up, for instance when a tankcontaining fuel is practically emptied and then filled with a fuelcontaining mainly methanol. Using the tank levels before and after tankfill, an assessment of what sort of fuel compositions can be present ismade on the basis of commercial fuel data. The input control values arethen modified for running the internal combustion engine using fuels ofthe possible compositions, and the system assesses which composition theinternal combustion engine runs best with. These values are used fromthen on for controlling the internal combustion engine.

The disadvantage of these known methods for determining the fuelquality, however, is that they are coupled with the combustion processand hence may also be affected by other parameters.

SUMMARY OF THE INVENTION

The invention relates to device for determining the quality of a fuelindependently of the combustion process and a corresponding method fordetermining the fuel quality.

The invention is based on the technical knowledge that the fuel qualitycan be assessed from its gas emission behavior. For instance a fuelcontaining a high proportion of long-chain hydrocarbons has a weaker gasemission than a fuel containing a high proportion of short-chainhydrocarbons, because long-chain hydrocarbons are less volatile. The gasemission behavior of the fuel therefore reflects the long-chain andshort-chain hydrocarbon composition of the fuel and hence the fuelquality.

It is therefore provided within the invention to measure the pressureand/or temperature in a fuel container in order to derive from this aquality value that represents the fuel quality.

The pressure or temperature in the fuel container is preferably measuredby a separate pressure sensor or temperature sensor respectively.Alternatively, however, the pressure and/or temperature in the fuelcontainer may be derived from other measured values, avoiding the needfor a separate pressure and/or temperature sensor. The term used in thecontext of the invention for a pressure and/or temperature sensor shouldtherefore be understood as a general term covering all arrangements inwhich the pressure and/or the temperature in the fuel container ismeasured directly or indirectly. For example, the temperature sensor mayalso measure the fuel temperature in the low-pressure section of theinjection system in order to derive the fuel temperature in the fuelcontainer.

In addition, the device according to the invention contains anevaluation unit whose inputs are connected to the temperature sensorand/or the pressure sensor, and which determines the quality value as afunction of the pressure and/or temperature in the fuel container.

The evaluation unit preferably contains two processing units, where thefirst processing unit is connected to the pressure sensor and thetemperature sensor and determines a gas emission characteristic valuerepresenting the gas emission behavior of the fuel as a function of thepressure and temperature in the fuel container.

The second processing unit then determines the quality value of the fuelon the basis of the gas emission characteristic value, the gas emissioncharacteristic value preferably being linked to the quality-valueaccording to a functional relationship. The second processing unit mayalso store this functional relationship in the form of a table.

The evaluation unit preferably contains a differentiator whichdetermines the rate of change in pressure in the fuel container. Theterm differentiator, however, should be understood in its general sensein the context of the patent, and covers not only differentiation in thenarrow mathematical sense, but also components or sub-assemblies thatmeasure the rate of change in pressure.

In the preferred embodiment of the invention, a comparator unit is alsoprovided, which compares the rate of change in pressure in the fuelcontainer with a preset threshold value. This procedure is based on theknowledge that the gas emission behavior of the fuel is highlytemperature-dependent and exhibits an abrupt change at a specificthreshold temperature. In fact when the temperature exceeds thethreshold temperature, the fuel gas emission increases abruptly, whichaccordingly causes an abrupt rise in pressure in the fuel container whenthe fuel warms up. In the opposite situation, during cooling, thepressure in the fuel container drops abruptly when the temperature fallsbelow the threshold temperature.

Thus in order to be able to determine the threshold temperature as thegas emission characteristic value, the preset threshold value for thechange in pressure preferably equals the change in pressure at thepreset threshold temperature.

Finally, in the preferred embodiment of the invention, a sample-and-holddevice is provided, which is triggered by the comparator unit to storetemporarily the temperature currently measured in the fuel containerwhen the current change in pressure in the fuel container exceeds thepreset threshold value. In this way the sample-and-hold device alwaysholds the threshold temperature at which the fuel abruptly changes itsgas emission behavior.

The method according to the invention for determining the fuel qualitycan also be implemented as a computer program, for instance by suitablemodification of the control program in the electronic control unit of aninjection system. The device according to the invention can accordinglybe fully integrated in the electronic control unit of an injectionsystem.

It is also possible, however, to design the device according to theinvention as a separate sub-assembly, which can then be connected viadata lines to the electronic control unit of the injection system.

In addition, it should be mentioned that the method according to theinvention can be implemented both after the internal combustion engineis switched off when the fuel is cooling and while the internalcombustion engine is running when the fuel is warming up. The onlycrucial factor is that the gas emission behavior of the fuel in the fuelcontainer varies according to the temperature, which enables anassessment of the fuel composition.

An advantage of the device according to the invention and thecorresponding method is the relatively low cost, because conventionalinjection systems usually already have a temperature sensor formeasuring the fuel temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages of the present invention will be described withreference to the following figures in which:

FIG. 1 shows a device according to the invention for determining thefuel quality,

FIG. 2 shows the operating method of the device of FIG. 1 as a flowdiagram, and

FIG. 3 a and 3 b show a cooling and heating curve for the fuel:

DETAILED DESCRIPTION OF THE INVENTION

The device according to the invention shown in FIG. 1 is used fordetermining the quality of the fuel in a fuel container of an internalcombustion engine and enables the determined fuel quality to be takeninto account when running the internal combustion engine, for example bycontrolling an injection system accordingly.

The determination of the fuel quality according to the invention isbased on the knowledge that the gas emission behavior of the fueldepends on the fuel quality, because long-chain hydrocarbons have aweaker gas emission than short-chain hydrocarbons. For a standardcomposition of the fuel containing a defined proportion of long-chainand short-chain hydrocarbons, the fuel therefore has a defined gasemission behavior, while deviations in the fuel composition from thestandard are associated with a modified gas emission behavior.

The gas emission behavior of the fuel is in this case measured on thebasis of the fuel held in the fuel container, the fuel container beingclosed off during the measurement so that the fuel gas emissions resultin a measurable pressure change.

Thus the device according to the invention contains a pressure sensor 1,which is arranged in the wall of the fuel container and measures thepressure difference Δp between the internal pressure p_(Internal) in thefuel container and the ambient pressure p_(External).

In addition, another pressure sensor 2 is provided, which measures theambient pressure p_(External) so that altitude or weather-relatedchanges in the ambient pressure p_(External) can be taken into account.The pressure sensor 2 may be a conventional sensor in the narrowersense, but it is also possible for the ambient pressure p_(External) tobe derived from other measured values.

The outputs of the two pressure sensors 1, 2 are connected to aprocessing unit 3, which calculates the internal pressure p_(Internal)in the fuel container from the pressure difference Δp and the ambientpressure p_(External).

The gas emission behavior of the fuel does not depend solely on thelong-chain and short-chain hydrocarbon composition of the fuel however,but also on the fuel temperature, because as the fuel temperatureincreases more gas is emitted by the fuel, leading to a correspondingincrease in pressure.

The device according to the invention therefore additionally contains atemperature sensor 4, which is also arranged in the wall of the fuelcontainer and measures the fuel temperature.

The outputs of the processing unit 3 and the temperature sensor 4 areconnected to an evaluation unit 5, which determines a quality value Qrepresenting the fuel quality from the measured values for pressure pand temperature T.

The evaluation unit 5 contains a processing unit 6 for this purpose,which determines the rate of change dp/dt in the pressure difference pmeasured by the pressure sensor 1.

The output of the processing unit 6 is connected to a further processingunit 7, which determines the absolute value of the rate of change inpressure. This is necessary because the rate of change in pressure dp/dtis negative when the fuel is cooling after switching off the internalcombustion engine, as shown in FIG. 3 a. In contrast, the rate of changein pressure dp/dt is positive when measured after starting up theinternal combustion engine, as shown in FIG. 3 b.

The output of the processing unit 7 is connected to a comparator unit 8,which compares the absolute value of the rate of change in pressuredp/dt with a preset threshold value dt/dt_(MIN).

This makes use of the knowledge that the fuel gas emission behavior ishighly temperature-dependent, which results in an abrupt pressure changeat a specific threshold temperature T_(TH), as can be seen in FIG. 3 aand 3 b. In this case the preset threshold value dp/dt_(MIN) issubstantially equal to the rate of change in pressure at the thresholdtemperature T_(TH), so that the comparator unit 8 enables the thresholdtemperature T_(TH) to be determined.

The threshold temperature T_(TH) is not constant, however, but dependson the fuel composition and reflects the gas emission behavior of thefuel. A low threshold value T_(TH) is an indicator of a highly volatilefuel containing a high proportion of short-chain hydrocarbons. A highthreshold value T_(TH), on the other hand, results from a highproportion of long-chain hydrocarbons, which means that the fuel is lessvolatile. The threshold value T_(TH) is hence a gas emissioncharacteristic value that reflects the gas emission behavior of thefuel.

Depending on the comparison result, the comparator unit 8 triggers asample-and-hold device 9, whose input is connected to the temperaturesensor 4. If the rate of change in pressure dp/dt exceeds the presetthreshold value dp/dt_(MIN), the sample-and-hold circuit 9 accepts andstores as the threshold temperature T_(TH) the temperature T currentlybeing measured. Otherwise the sample-and-hold device 9 retains thepreviously measured value for the threshold temperature T_(TH).

Finally, the sample-and-hold device 9 is connected to a processing unit10, which calculates the quality value Q representing the fuel qualityfrom the stored gas emission characteristic value T_(TH). The qualityvalue is calculated according to a defined functional relationship,which is saved in the form of a table in the processing unit 10.

An injection system for an internal combustion engine can then take intoaccount the quality value Q determined by the device according to theinvention in order to compensate for the effects of a varying fuelquality.

The method according to the invention is described below with referenceto the flow diagram shown in FIG. 2.

After the start, the temperature T and the internal pressurep_(Internal) in the fuel container of the internal combustion engine aremeasured continuously in a loop.

The pressure gradient dp/dt is also calculated continuously and theabsolute value of the pressure gradient determined continuously.

The absolute value of the pressure gradient is then comparedcontinuously with the preset threshold value dp/dt_(MIN), this being therate of change in pressure that occurs during the abrupt change in thefuel gas emission behavior at the threshold temperature T_(TH),corresponding to the time t_(TH) in FIGS. 3 a and 3 b.

If the pressure gradient dp/dt exceeds the preset threshold valuedp/dt_(MIN), the current temperature T is stored as the thresholdtemperature T_(TH). Otherwise the process continues in the loop untilthe current pressure gradient dp/dt exceeds the threshold valuedp/dt_(MIN).

In a final step, the quality value Q representing the fuel quality isthen calculated from the stored threshold temperature T_(TH) accordingto a defined functional relationship. The injection system can then takeinto account this quality value Q in order to compensate for the effectof a varying fuel quality.

The method according to the invention described above works both as thefuel is cooling after switching off the internal combustion engine andwhen the engine is running with the fuel warming up slowly.

FIG. 3 a shows the behavior of the temperature T and the pressure p inthe fuel container as the fuel cools after switching off the internalcombustion engine, when the temperature T declines exponentially. Thepressure p also follows the downward temperature curve, but the pressurecurve exhibits an abrupt drop at the time t_(TH), when the temperaturefalls below the threshold temperature t_(TH). The reason for this isthat the fuel gas emission behavior is highly temperature-dependent andemission sets in at the threshold temperature T_(TH). The thresholdtemperature T_(TH) thus constitutes a gas emission characteristic valuerepresenting the gas emission behavior of the fuel.

FIG. 3 b, on the other hand, shows the behavior of the temperature T andthe pressure p in the fuel container after starting up the internalcombustion engine, when the fuel temperature T increases exponentially.The pressure p in the fuel container also increases with thetemperature, and exhibits an abrupt rise when the temperature exceedsthe threshold temperature T_(TH). The reason for this is that the fuelgas emission behavior increases abruptly when the threshold temperatureT_(TH) is exceeded, so that the threshold temperature T_(TH) constitutesa gas emission characteristic value representing the fuel gas emissionbehavior that is not temperature-dependent.

The invention is not restricted to the exemplary embodiment describedabove, but one can think of a multiplicity of variations andmodifications that also make use of the idea of the invention and hencefall within the scope of protection.

1. A device for determining the quality of fuel for an internalcombustion engine, comprising: a pressure sensor for measuring thepressure in a fuel containers; and a temperature sensor for measuringthe temperature in a fuel container; and an evaluation unit with inputsthat are connected to the pressure sensor and the temperature sensor,for determining a quality value representing the fuel quality, whereinthe evaluation unit determines the quality value as a function of thetemperature and the pressure in the fuel container in that theevaluation unit, derives the quality value therefrom.
 2. The device asclaimed in claim 1, wherein the evaluation unit comprises a firstprocessing unit which has inputs that are connected to the pressuresensor and the temperature sensor and which determines, as a function ofthe pressure and temperature in the fuel container, a gas emissioncharacteristic value representing the gas emission behavior of the fuel,and the evaluation unit comprises a second processing unit which has aninput that is connected to the first processing unit and whichdetermines the quality value of the fuel as a function of the gasemission characteristic value.
 3. The device as claimed in claim 2,wherein the first processing unit comprises a differentiator whichdetermines the rate of change in the pressure in the fuel container. 4.The device as claimed in claim 3, wherein the first processing unitcomprises a comparator unit which has inputs that are connected to thedifferentiator and which compares the rate of change in pressure in thefuel container with a preset threshold value.
 5. The device as claimedin claim 4, wherein the evaluation unit comprises a sample-and-holddevice having a sampling input and a control input, the sampling inputbeing connected to the temperature sensor, while the control input isconnected to the comparator unit.
 6. A method for determining thequality of fuel for an internal combustion engine, comprising: measuringpressure and/or temperature in a fuel container, while the fuel is in afuel container; and determining a quality value representing the fuelquality, wherein the quality value is determined as a function of themeasured temperature and the measured pressure in the fuel container inthat the quality value is derived therefrom.
 7. The method as claimed inclaim 6, further comprising: determining a gas emission characteristicvalue, representing the gas emission behavior of the fuel as a functionof the temperature and the pressure in the fuel container; anddetermining the quality value of the fuel as a function of the gasemission characteristic value determined for the fuel.
 8. The method asclaimed in claim 7, further comprising: determining the rate of changein pressure in the fuel container; and determining the gas emissioncharacteristic value as a function of the rate of change in pressure inthe fuel container.
 9. The method as claimed in claim 8, furthercomprising: comparison of the rate of change in pressure in the fuelcontainer with a preset threshold value; and determining the gasemission characteristic value as the temperature in the fuel containerat which the preset threshold value for the change in pressure isreached or exceeded.
 10. The method as claimed in claim 6, wherein thefuel container is closed off during measurement of the pressure and thetemperature.
 11. The method as claimed in claim 10, wherein the fuelcontainer has tank ventilation that is shut off during measurement ofthe pressure in the fuel container.
 12. The method as claimed in claim6, wherein the internal combustion engine is switched off duringmeasurement of the pressure in the fuel container.
 13. The method asclaimed in claim 6, wherein the fuel is injected into a combustionchamber of an internal combustion engine as a function of the qualityvalue.